5Factors causing congestionStreams of packets simultaneously begin arriving on several input lines that are headed for the same output lineSlow processorsLow-bandwidth linesMismatch between parts of the systemFeeding congestion upon itselfRetransmission of lost packetsTUNALIComputer Networks 1

6Congestion Control versus Flow ControlMaking sure that the net is able to carry the offered trafficFlow controlRelated to point-to-point traffic between the sender and the receiverMaking sure that a fast sender can not continually transmit data faster than the receiver can absorb itA sender may receive a ‘slow down’ message either due tocongestion in the subnet ordue to receiver not being able to handle the loadTUNALIComputer Networks 1

15Choke PacketsIf a newly arrived packet’s output line is in warning state,a CHOKE PACKET is sent back to the sourceThe packet is forwarded to the destinationThe source receiving the choke packet is expected to reduce its traffic accordinglyTUNALIComputer Networks 1

21Load Shedding 3Random dropping can be done but there are better policiesWine policyOld is better than newMore efficient in go back nMilk policyNew is better than oldMore efficient in multimedia streamingPriority policyMPEG uses compression that has some frames carrying more important informationTUNALIComputer Networks 1

22Random Early DetectionDiscard packets before all the buffer space is really exhaustedThis would be a message to TCP to slow downMaintain a buffer threshold and when average queue length exceeds the threshold, action is takenTUNALIComputer Networks 1

27Traffic Shaping 1 Bursty traffic is often the cause of congestionTraffic shaping regulates the average rate (and burstiness) of data transmissionThis is an open loop method to prevent congestion.When virtual circuit is set up, the user an d the subnet agree on certain traffic pattern.As long as the user fulfills its contract, the carrier live up to its promiseTraffic policing is the monitoring of user’s rateCarrier discards packets if the user breaks the contractTUNALIComputer Networks 1

30The Leaky Bucket AlgorithmPackets enter a finite queue at a random ratePackets leave the queue at a constant rateIf the buffer is full, the incoming packets are discardedIf packets are of variable length, then the algorithm is defined in terms of bytes per unit timeTUNALIComputer Networks 1

31The Token Bucket AlgorithmA leaky bucket holds tokens that are generated at rate of 1 token in T sec.For a packet to be transmitted, it must capture and destroy one token.This algorithm allows output bursts of up to bucket capacity.This algorithm discards tokens if the bucket is full, but it never discards packets, hence needs a large buffer for incoming packetsA variant of the algorithm running on bytes rather than packets is possibleTUNALIComputer Networks 1

42RSVP 2 Receivers send a reservation message up the tree to the senderMessage propagates by reverse path forwardingAt each hop, the router reserves the necessary bandwidthIf there is no available bandwidth, it reports back failureBy the time message arrives to the source, bandwidth has already been reservedTUNALIComputer Networks 1

43Differentiated ServicesManaging thousands of flows is a difficult taskAdvance setup is requiredDifferentiated services allow definition of a set of service classesCustomers sign up for a particular class and receive the appropriate serviceNo advance setup is requiredTUNALIComputer Networks 1

44Expedited Forwarding 1 Two classes of service are available RegularThe routers could be programmed to have two output queues for each outgoing line to servive the above classesTUNALIComputer Networks 1